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        // 1.弹窗
        alert('哈喽')
        // 2.控制台输出
        console.log('123')
        // 3.在页面输出内容
        document.write('123')
        // 4.输入
        prompt('请输入')
         
        // 案例:计算圆的面积
        let r = prompt('请输入圆的半径')
        let π = 3.14
        let area = π*r*r
        document.write(area)
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